The present invention generally relates to equipment for seismic exploration of a subterranean formation, more particularly to an apparatus for storing control units.
For simplicity, numerous details known in the art will be omitted from the following description. However, these details must of course be present in practical implementations.
A conventional seismic survey at sea is performed from a source vessel towing a set of acoustic sources, for example airguns, which are set off at regular intervals in so-called shots. Acoustic waves from the airguns travel through the water into a subterranean formation, where they are reflected and refracted from various strata in the formation. The reflected and refracted waves are detected by sensors and recorded for later analysis in order to provide information about the subterranean structure or formation.
The reflected and refracted waves comprise compression or pressure waves, shear waves, Stoneley waves etc., and are typically detected by different sensors such as geophones and hydrophones.
In order to achieve as much information as possible, the sensors are typically deployed in close acoustic contact with the seafloor. Furthermore, the sensors are conventionally arranged in nodes, each node comprising one or more seismic sensors. The nodes may be deployed or planted one by one, e.g. by an ROV, or they may be connected by an Ocean Bottom Cable (OBC), conventionally at intervals of 25 or 50 meters. Today, methods using OBCs can be loosely divided into to different groups.
The first group comprises methods wherein the nodes transmit seismic signals to the exploration vessel in real time. A first example of methods in this group can be found in U.S. Pat. No. 4,942,557 to Seriff, wherein an airgun generates compression waves and wherein converted shear waves from the formation are detected and corresponding signals are transmitted over a wire to an exploration vessel. A second example can be found in U.S. Pat. No. 4,780,863 to Schoepf wherein the seismic signal is converted to an electrical signal carried over a wire to a buoy on the surface where it is converted to a radio signal and transmitted to the exploration vessel in real-time.
Major shortcoming of the methods from the first group are (a) increased complexity of an ocean bottom cable which must comprise conducting wires as well as protection against stress and abrasion that might harm the conducting wires, (b) increased probability of water leakage through the multiple electrical terminations connecting wires to the electronics within each node and (c) rapidly increasing cost with increasing depth of deployment. The increasing cost is partly due to the length of a cable with a relatively high cost per unit length, and partly due to increased sealing requirements when the depth increases.
The second group comprises methods employing completely autonomous nodes which are left on the seafloor for the duration of a survey, i.e. the period of time during which the data from a series of shots are recorded and stored. After the survey, the autonomous nodes are retrieved to a recording vessel where the recorded seismic data are transferred from the nodes for later analysis. These methods reduce acoustic noise induced by a cable extending from the seafloor to the surface, and they tend to be advantageous at greater deeps, as the seismic sensors, associated electronics and power source can be deployed in a pressure tight housing 24 or shell without conduits for wires that require sealing.
The present disclosure concerns the second group involving autonomous nodes, i.e. methods wherein each node has a separate power source and means for storing data obtained from a survey during which the node resides incommunicado on the seafloor.
US 2013/0058192 A1 to Gateman et al. and assigned to the applicant for the present invention discloses an ocean bottom seismic cable comprising a plurality of seismic node casings separated by stress member sections with acoustic de-couplers, such that the nodes are deployed at predetermined intervals, typically 25 to 50 meters, and such that each node receives a practical minimum of noise from adjacent nodes. Each node casing comprises an autonomous sensor capsule that can be inserted as a unit into a seismic node casing during deployment from a vessel and be removed as a unit from a seismic node casing when the cable is retrieved to the vessel.
The removable sensor capsule is a container made from e.g. steel or titanium that can withstand the pressure at the seafloor. During operation it contains the sensors required to detect the seismic signals and other parameters of interest, at least one battery unit for power supply and at least one control unit comprising hardware, firmware and software required for recording and storing the seismic data obtained during a survey until the control unit is retrieved to the exploration vessel and the seismic data are uploaded for further analysis. Some signal processing may also be performed by the control unit.
The battery unit and control unit are conveniently releasable connected to form a control/battery-unit. US 2013/0058192 A1 discloses an embodiment in which two control/battery-units are redundantly disposed at opposite ends of the sensor capsule.
Consider next the deck of an exploration vessel for performing a seismic survey. Space is at a premium, and must be provided for drums for various cables such as streamers containing acoustic sources and ocean bottom cables comprising SSRs. Space is also required for equipment used to deploy and retrieve the various cables, e.g. one or more winches, ROVs, cranes etc. In addition, batteries may be considered a safety risk and for this reason regulations may require a separate storage space for batteries.
Thus, there is a general need for effective use of space on the deck of a survey vessel. In particular, there is a need for effective handling of sensor capsules aboard the vessel before they are deployed and after they are retrieved.
[Application, inventors Jan Gateman & Nils Gateman] assigned to the applicant for the present invention, discloses a method for handling sensor capsules in which assembly involves the steps of connecting a control unit to a battery unit, preferably by relative axial and rotational motions between the battery unit and the control unit, and inserting the resulting control/battery unit into a sensor capsule. The application further discloses automated disassembly of the sensor module, including storing the battery unit, the control unit and the sensor capsule in drawers and/or drawers that are subsequently inserted into cabinets. A docking station for control units is also disclosed. The docking station comprises several drawers, each with several docking sockets—one docking socket per control unit. An automated assembly inserts the control units into a docking socket by a combination of axial motion and rotation and removes the battery unit after the control unit is connected to a power source through the docking socket. Thus, the control unit conveniently has one power inlet from the battery unit at a first end, and a second power inlet at an opposite end for power supply through the docking socket.
The present invention relates to a docking station for use in a method disclosed in [Application, inventors Jan Gateman & Nils Gateman]. For convenience, the automated assembly used to insert and withdraw control units from the docking sockets is termed a “robotic gripper” in the following. From the description above, it is understood that the robotic gripper is able grip a generally cylindrical item, to position the item over a docking socket, to move the item axially, and to rotate the item about its cylinder axis.
The control unit may comprise a CPU capable of executing instructions in software, e.g. in order to process the detected seismic signals before they are stored. Similar tasks can be performed by hardware, for example implemented in programmable arrays of logic gates, and the control unit may hence comprise programmable hardware. Further, the control unit may contain firmware, timers and other components known in the art. Software, firmware and programmable hardware require tests to insure their integrity and functionality, e.g. reading a version number. Furthermore, the software, firmware and programmable hardware may require an update or upgrade from time to time. The above and similar tasks are referred to as “test and maintenance” in the following.
The docking station disclosed in [Application, inventors Jan Gateman & Nils Gateman] comprises power supply and capabilities for uploading data from all control units inserted into it. The docking station does not cover the need for test and maintenance, e.g. calibrating sensors, updating software etc. Furthermore, keeping all control units electrically connected in docking sockets at all times is expensive.
The objective of the present invention is to solve or alleviate at least one of the problems above while keeping the benefits of prior art.